Ember not setting range slider value

In a form, in a table, I have...

<tr><td>Score (-10 to +10):</td><td><br/>Score: </td></tr>

This almost works; the correct value is reported below the slider, and any value set using the slider is stored correctly. But the slider is not initialized with the value correctly. If I replace the binding name with a constant (e.g. "7"), that does initialize properly.

This looks like an Ember bug, in that this same format works for other input types and it does report it correctly; it's just the initialization that's wrong. But it seems kind-of big to be wrong, so there must be some secret magical incantation experienced users know, that I'm just ignorant of. How do I get this to initialize?